Programmable resistance memory element with threshold switching material

ABSTRACT

A programmable resistance memory element comprising a dielectric material between a programmable resistance memory material and a threshold switching material.

FIELD OF THE INVENTION

The present invention relates generally to electrically programmablephase-change memory. More specifically, the present invention relates toan electrically programmable phase change memory that includes athreshold switching material.

BACKGROUND OF THE INVENTION

The use of electrically programmable phase-change materials (forexample, materials which can be electrically programmed betweenamorphous and crystalline states) for electronic memory applications iswell known in the art and is disclosed, for example, in commonlyassigned U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205,5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 thedisclosures of which are all incorporated by reference herein. Stillanother example of a phase-change memory element is provided in commonlyassigned U.S. patent application Ser. No. 09/276,273, the disclosure ofwhich is incorporated by reference herein.

Generally, phase-change materials are capable of being electricallyprogrammed between a first structural state where the material isgenerally amorphous and a second structural state where the material isgenerally crystalline. The term “amorphous”, as used herein, refers to acondition which is relatively structurally less ordered or moredisordered than a single crystal. The term “crystalline”, as usedherein, refers to a condition which is relatively structurally moreordered than amorphous. The phase-change material exhibits differentelectrical characteristics depending upon its state. For instance, inits crystalline, more ordered state the material exhibits a lowerelectrical resistivity than in its amorphous, less ordered state.

Materials that may be used as a phase-change material include alloys ofthe elements from group VI of the Periodic Table. These group VIelements are referred to as the chalcogen elements and include theelements Te and Se. Alloys that include one or more of the chalcogenelements are referred to as chalcogenide alloys. An example of achalcogenide alloy is the alloy Ge₂Sb₂Te₅.

FIG. 1 is an example of a plot of the resistance of a volumechalcogenide phase-change memory material versus the amplitude of acurrent pulse through the memory material. Referring to FIG. 1, severaldifferent programming regimes can be distinguished. In the left side ofthe curve, the resistance of the memory material remains substantiallyconstant (i.e., in its high resistance or RESET state) until a currentpulse of sufficient energy is applied. The memory material is thentransformed from its high resistance (RESET) state to its low resistance(SET) state. The current pulse sufficient to program the memory materialfrom the high resistance state to the low resistance state is referredto as a “set pulse”. While not wishing to be bound by theory, it isbelieved that the set pulse is sufficient to change at least a portionof the volume of memory material from a less-ordered amorphous state toa more-ordered crystalline state.

The volume of memory material may be programmed back from the lowresistance state or SET state to the high resistance or RESET state byapplying a current pulse of sufficient amplitude, referred to as a“reset pulse”. While not wishing to be bound by theory, it is believedthat application of a reset pulse to the memory material is sufficientto change at least a portion of the volume of memory material from amore-ordered crystalline state to a less-ordered amorphous state. Thememory material may be programmed back and forth between the highresistance or RESET state and the low resistance or SET state. This typeof programming scheme provides for a binary mode of operation (forexample, the RESET state may be a logic 0 while the SET state may be alogic 1).

Referring to the right side of the curve of FIG. 1, as the amplitude ofthe current through the memory material increases, the resistance of thedevice increases. This increase is both gradual and reversible. In thisregime, the chalcogenide memory material may be programmed to anyresistance value within a window of resistance values bounded by the lowresistance or SET state and the high resistance or RESET state. Morespecifically, in this regime along the right side of the curve, thememory element may be programmed from any one of the resistance stateson the right side of the resistance curve to any other of the resistancestates on the right side of the curve by the application of a currentpulse of sufficient amplitude. The memory material may thus beprogrammed between three or more resistance values within the resistancewindow so as to provide for multi-state, directly overwritable datastorage. While not wishing to be bound by theory, it is believed thateach of the resistance states along the right side of the curve maycorrespond to a particular ratio of the volume of crystalline materialto the volume of amorphous material in an active region of thechalcogenide material. Three intermediate resistance states R1, R2 andR3 are shown in the resistance curve of FIG. 1.

SUMMARY OF THE INVENTION

An aspect of the present invention, an electrically programmable memoryelement, comprising: a programmable resistance material; a thresholdswitching material; and a first layer of a dielectric material betweenthe programmable resistance material and the threshold switchingmaterial.

Another aspect of the invention is a programmable resistance memoryelement, comprising: a programmable resistance material; a dielectricmaterial formed over the programmable resistance material; and athreshold switching material formed over the dielectric material.

Another aspect of the invention is a programmable resistance memoryelement, comprising: a threshold switching material; a dielectricmaterial formed over the threshold switching material; and aprogrammable resistance material formed over the dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a resistance curve of a volume ofchalcogenide memory material;

FIG. 2 is an example of a current-voltage characteristic curve for achalcogenide threshold switching material;

FIG. 3 is an embodiment of a memory element of the present inventionincluding a breakdown layer;

FIG. 4 is the embodiment of the memory element shown in FIG. 3 with ahole formed in the breakdown layer;

FIG. 5 is an embodiment of a memory element of the present inventionincluding a first and a second breakdown layer;

FIG. 6 is the embodiment of the memory element shown in FIG. 5 withholes formed in the first and second breakdown layers;

FIG. 7 is an embodiment of a memory element of the present inventionincluding a first and a second breakdown layer;

FIG. 8 is the embodiment of the memory element shown in FIG. 7 withholes formed in the first and second breakdown layers;

FIG. 9 is an embodiment of a memory element of the present inventionincludes a first, second and third breakdown layer; and

FIG. 10 is the embodiment of the memory element shown in FIG. 9 withholes formed in the first, second and third breakdown layers.

DETAILED DESCRIPTION OF THE INVENTION

In the following paragraphs and association with the accompanyingfigures, examples of memory elements formed according to embodiments ofthe invention are presented. Specific embodiments of the memory elementsand methods of making such memory elements are described as they mightbe implemented for use in semiconductor memory circuits. In interest ofclarity, not all features of an actual implementation are described inthis specification.

Turning now to the drawings, an embodiment of the present invention isshown in FIG. 3. FIG. 3 shows a memory element 100. A semiconductorsubstrate 110 is provided. The substrate 110 may include bitlines aswell as wordlines used to access the memory element. A layer 120 of aconductive material is formed over the substrate 110. The layer 120 ofconductive material forms a first electrode for the memory element (inthis case the first electrode is a bottom electrode). The conductivematerial used for layer 120 may be any conductive material. Examples ofconductive materials include titanium-tungsten, tungsten, tungstensilicide, molybdenum, titanium nitride, titanium carbon-nitride,titanium aluminum-nitride, titanium silicon-nitride, and carbon. Otherexamples of conductive materials include n-type doped polysilicon,p-type doped polysilicon, p-type doped silicon carbon alloys and/orcompounds, n-type doped silicon carbon alloys.

A layer 130 of a programmable resistance material is deposited over thelayer 120 of conductive material. The programmable resistance materialserves as the memory material for the memory element 100. Theprogrammable resistance material includes any material which iselectrically programmable between at least a first and a secondresistance state. Preferably, the programmable resistance material ischosen to be a phase change material. The phase change material ispreferably a chalcogenide alloy that includes at least one chalcogenelement. An example of a chalcogenide phase-change material suitable foruse as a memory material is the chalcogenide alloy Ge₂Sb₂Te₅. The memorymaterial 120 is preferably deposited in its low resistance state. Hence,when the memory material is a chalcogenide phase change material, thechalcogenide alloy is preferably deposited in its crystalline state.

A breakdown layer 140 is then deposited over the memory material 120.Preferably, the breakdown layer 140 shown in FIG. 3 as well as all otherbreakdown layers described herein are formed of a dielectric material.Examples of dielectric materials which may be used include nitrides(such as silicon nitride) and oxides (such as silicon oxide). It ispossible that mixtures of oxides and nitrides may also be used. If twoor more breakdown layers are used in an embodiment of a memory elementof the present invention, each of the breakdown layers may be formed ofthe same material or it is possible that different materials may be usedfor two or more of the breakdown layers.

The breakdown layer 140 as well as all other breakdown layers disclosedherein preferably have a thickness which is less than about 100Angstroms, more preferably less than about 75 Angstroms, and mostpreferably less than about 50 Angstroms. In one embodiment of theinvention one or more of the breakdown layers may have a thicknessbetween about 20 Angstroms and about 50 Angstroms. If two or morebreakdown layers are used, all may have substantially the samethickness, or two or more of the breakdown layers may have differentthicknesses.

A layer 150 of a threshold switching material is then deposited over thebreakdown layer 140. The threshold switching material used is preferablya chalcogenide threshold switching material. An example of achalcogenide alloy which may be used as a threshold switching materialis the alloy Si₁₄Te₃₉As₃₇Ge₉X₁ where X may be element In or the elementP.

A layer 160 of a conductive material is deposited over the thresholdswitching material 150. The layer 160 serves as the second or topelectrode of the memory element 100. Generally, any conductive materialmay be used for layer 160. Examples of conductive materials which may beused for layer 160 include those listed above for layer 120. Theconductive materials used for first and second electrodes 120, 160 maybe the same materials or different materials.

As noted, the threshold switching material 150 is preferably achalcogenide threshold switching material. Associated with achalcogenide threshold switching material is a current-voltage, or“I-V”, characteristic curve. The I-V characteristic curve describes therelationship between the current through the threshold switchingmaterial as a function of the voltage across the material.

An example of an I-V characteristic curve for a chalcogenide thresholdswitching material is shown in FIG. 2. FIG. 2 shows the I-V plot in boththe first quadrant (where voltages and currents are positive) and thethird quadrant (where voltages and currents are negative). While onlythe first quadrant is described below, an analogous description appliesto the curve in the third quadrant of the I-V plot (where the voltageand the current are both negative).

The I-V characteristic curve IV includes an “off-state” branch 50 and an“on-state” branch 60. The off-state branch 50 corresponds to the branchin which the current passing through the threshold switching materialincreases slightly upon increasing the voltage applied across thethreshold material. This branch exhibits a small slope in the I-V plotand appears as a nearly horizontal line in the first (and third)quadrant of FIG. 3. The on-state branch 60 corresponds to the branch inwhich the current passing through the threshold material increasessignificantly upon increasing the voltage applied across the thresholdmaterial. The magnitude of the slope of the on-state branch is greaterthan the magnitude of the slope of the off-state branch. In the exampleshown in FIG. 2, the on-state branch exhibits a large slope in the I-Vplot and appears as a substantially vertical line in the first (andthird) quadrant of FIG. 2. The slopes of the off-state and on-statebranches shown in FIG. 2 are illustrative and not intended to belimiting. Regardless of the actual slopes, the on-state branch exhibitsa steeper slope than the off-state branch. When conditions are such thatthe current through the threshold material and voltage across materialis described by a point on the off-state branch of the I-V curve, thethreshold material is said to be in the “OFF” state.

Likewise, when conditions are such that the current through thethreshold material and voltage across the threshold material isdescribed by a point on the on-state branch of the I-V curve, thethreshold material is said to be in the “ON” state.

The switching properties of the threshold switching material can bedescribed by reference to FIG. 2. When no voltage is applied across thematerial, the material is in the “OFF” state and no current flows. Thiscondition corresponds to the origin of the I-V plot shown in FIG. 2(current=0, voltage=0). The threshold switching material remains in theOFF state as the voltage across the threshold switching material and thecurrent through the threshold switching material is increased, up to avoltage V_(t) which is referred to as the “threshold voltage” of thethreshold switching material. When the voltage across the thresholdswitching material is less than V_(t), the slope of the off-state branchof the I-V curve is small and the current flowing through the thresholdswitching material increases only in a small amount as the appliedvoltage is increased.

When the applied voltage across the threshold switching material equalsor exceeds the threshold voltage V_(t), the threshold switching materialswitches from the off-state branch 50 to the on-state branch 60 of theI-V curve. The switching event occurs instantaneously and is depicted bythe dashed line in FIG. 2. Upon switching, the voltage across thethreshold switching material decreases significantly and the currentthrough the threshold switching material becomes much more sensitive tochanges in the device voltage (hence, branch 60 is steeper than branch50). The threshold switching material remains in the on-state branch 60as long as a minimum current, labeled I_(h) in FIG. 2, is maintained.I_(h) is referred to as the holding current of the threshold switchingmaterial and the associated voltage V_(h) is referred to as the holdingvoltage of the threshold switching material. If the threshold materialconditions are changed so that the current becomes less than I_(h), thethreshold switching material normally returns to the off-state branch 50of the I-V plot and requires re-application of a voltage which isgreater than or equal to the threshold voltage V_(t) to resume operationon the on-state branch. If the current is only momentarily (a time lessthan the recovery time of the chalcogenide material) reduced belowI_(h), the ON state of the memory element may be recovered uponrestoring the current through the memory element which is at or aboveI_(h).

Analogous switching behavior occurs in the third quadrant of the I-Vplot shown in FIG. 2. Provided one is aware of the negative polaritiesof both the voltage and current of the I-V curve in the third quadrant,the switching behavior in the third quadrant is analogous to thatdescribed hereinabove for the first quadrant. For example, appliedvoltages having a magnitude greater than the magnitude of the negativethreshold voltage in the third quadrant induce switching from theoff-state branch 50 to the on-state branch 60.

It is noted that the current-voltage characteristic curve shown in FIG.2 is an example of an “S-type” current-voltage characteristic curve. Itis possible that any threshold switching material that exhibits similarS-type current-voltage characteristics may be used as the thresholdswitching material in the memory element of the present invention.

Hence, as described above, the threshold switching material may beswitched from an OFF state to an ON state by application of a voltageacross the memory element having a magnitude which is greater than orequal to the magnitude of the threshold voltage V_(t). While not wishingto be bound by theory, it is believed that application of a voltageacross the threshold switching material which is at or above thethreshold voltage may cause the formation of a conductive channel orfilament within the threshold switching material. At the thresholdvoltage V_(t), the electric field experienced by the chalcogenidethreshold switching material is sufficiently high to induce a breakdownor avalanche effect whereby electrons are removed from atoms to form ahighly conductive, plasma-like filament of charge carriers. Rather thanbeing bound to atoms, some electrons become unbound and highly mobile.As a result, a conductive channel or filament forms. The conductivefilament constitutes a conductive volume within the otherwise resistivechalcogenide threshold switching material. The conductive filamentextends through the chalcogenide threshold switching material andprovides a low resistance pathway for electrical current. Portions ofthe chalcogenide material outside of the filament remain resistive.Since electric current traverses the path of least resistance, thepresence of a conductive filament renders the chalcogenide material moreconductive and establishes an “ON” state. The creation of a conductivefilament is the event that underlies the switching of the thresholdswitching material from its OFF state to its ON state.

Referring again to FIG. 3, when a voltage is applied across the memoryelement 100 by the first and second electrodes 120, 160, a significantportion of the applied voltage appears across the threshold switchingmaterial 150. (As noted above, the memory material 130 is preferablydeposited in its low resistance state so that the voltage drop acrossthe memory material 130 is thus very small and most of the voltage dropoccurs across threshold material 150).

When the voltage across the threshold switching material 150 reaches thethreshold voltage, the threshold switching material switches to its “ON”state and a conductive filament is created. While not wishing to bebound by theory, it is believed that the formation of the conductivefilament, in turn, may cause a small region of the breakdown layer 140to disintegrate so as to form a hole 170 through the breakdown layer 140as shown in FIG. 4. The size of the hole (e.g., the diameter of thehole) may be defined by the electrical current applied to the memoryelement when forming the filament divided by the saturation currentdensity in the conductive filament. Hence, the size of the hole may becontrolled by the magnitude of the current applied when the hole isformed. Preferably, it is through this hole 170 that substantially allof the programming current of any subsequent memory cycling pulses(e.g., set and reset pulses) may pass. That is, when subsequentprogramming currents are applied to the memory element 100 (by theelectrodes 120, 160), substantially all of these currents willpreferably pass through the hole 170. Hence, the programming currentdensity may be made to be relatively high in the portion of memorymaterial 130 that is in the vicinity of the hole 170. The overall amountof programming current needed to program the memory element may thus bereduced. The use of a breakdown layer 140 between the memory material130 and the threshold switching material 150 may thus serve to reducethe wasted current or leakage current of the memory element when thememory element is programmed.

An alternate embodiment of a memory element of the present invention isshown in FIG. 5. In this case, a first breakdown layer 140A is disposedbetween the memory material 130 and the threshold switching material 150while a second breakdown layer 140B is disposed between the thresholdswitching material 150 and the upper conductive layer 160. Each of thebreakdown layers is preferably formed of a dielectric material. Thedielectric material may be an oxide (such as a silicon oxide) or anitride (such as a silicon nitride). The breakdown layers 140A, 140B maybe formed of the same or different dielectric materials. Again, whilenot wishing to be bound by theory, it is believed that application of asufficient voltage across the memory element 100 causes the thresholdswitching material 150 to switch into the “ON” state and a conductivefilament to form. In addition, it is believed that the formation of theconductive filament may cause the disintegration of regions in both thefirst breakdown layer 140A and second breakdown layer 140B, therebycausing the formation of holes 170A and 170B in the first and secondbreakdown layers, respectively, as shown in FIG. 6. When a current isapplied to the memory element 100, substantially all of this currentpreferably flows through the holes 170A and 170B. Because of the smallsize of the holes 170A and 170B, the current density may be very high inthe portions of the memory material 130 and threshold switching material150 that are in the vicinity of the holes 170A, 170B.

Another embodiment of a memory element of the present invention is shownin FIG. 7. In this embodiment a first breakdown layer 140A is disposedbetween first conductive layer 120 and memory material 130, and a secondbreakdown layer 140B is disposed between the memory material 130 and thethreshold material 150. As described above, each of the breakdown layers140A, 140B is preferably formed of a dielectric material (such as anoxide or a nitride). Again, while not wishing to be bound by theory, itis believed that application of the appropriate voltage across thememory element may cause the formation of hole 170A in the firstbreakdown layer 140A and the formation of hole 170B in the secondbreakdown layer 140B as shown in FIG. 8. When a programming current isapplied to the memory element, it may be channeled through these holes.

Yet another embodiment of the invention is shown in FIG. 9. In thisembodiment a first breakdown layer 140A is disposed between the firstconductive layer 120 and the memory material 130, a second breakdownlayer 140B is disposed between the memory material 130 and the thresholdswitching material 150, and a third breakdown layer 140C is disposedbetween the threshold material 150 and the second conductive layer 160.Each of the breakdown layers may be formed of a dielectric material(such as an oxide or a nitride). Again, while not wishing to be bound bytheory, it is believed that application of the appropriate voltageacross the memory element may cause the formation of holes 170A, 170Band 170C in the breakdown layers 140A, 140B and 140C, respectively, asdepicted in FIG. 10.

It is to be understood that the disclosure set forth herein is presentedin the form of detailed embodiments described for the purpose of makinga full and complete disclosure of the present invention, and that suchdetails are not to be interpreted as limiting the true scope of thisinvention as set forth and defined in the appended claims.

1. An electrically programmable memory comprising: a programmableresistance material; a threshold switching material comprising achalcogen element; and a first layer of a dielectric material betweensaid programmable resistance material and said threshold switchingmaterial, said dielectric material including silicon nitride.
 2. Thememory of claim 1, further comprising a second layer of a dielectricmaterial, said threshold switching material being between said firstlayer and said second layer.
 3. The memory of claim 2, furthercomprising a third layer of a dielectric material, said programmableresistance material being between said third layer and said first layer.4. The memory of claim 1, further comprising a second layer of adielectric material, said programmable resistance material being betweensaid first layer and said second layer.
 5. The memory of claim 1,wherein said programmable resistance material is a phase-changematerial.
 6. The memory of claim 1, wherein said programmable resistancematerial comprises a chalcogen element.
 7. The memory of claim 1,wherein said first layer has a thickness of less than 100 Angstroms.